1. Field of the Invention
The present invention is directed toward unique graphene particulates and a process for the high-throughput production of such particulates, especially graphene nanoribbons (GNR) and graphene quantum dots (GQD). The process permits a high level of control over the width and crystallographic orientation of the graphene particulates produced. These graphene particulates may be used in a number of applications including, but not limited to, high speed transistors, electronic devices, sensors (biological, physical, explosive etc), solar cells, optoelectronic devices, logic devices, ultra-capacitors, actuators, coulomb blockade device, ultra-strong paper, field emission devices, transparent electrodes, conductive pastes, and optical/fluorescent devices.
2. Description of the Prior Art
Generally, graphene is a monolayer of carbon atoms tightly packed into a two-dimensional honeycomb lattice and is a basic building block for graphitic materials of all other dimensionalities. Graphene nanoribbons are single atom thick strips of sp2 hybridized carbon atoms that exhibit width and edge dependent band gap, scalar potential-mediated coupling of states in distinct bands, and room temperature ballistic transport.
In solid state physics, a band gap, also called an energy gap, is an energy range in a solid where no electron states can exist. In graphs of the electronic band structure of solids, the band gap generally refers to the energy difference (in electron volts) between the top of the valence band and the bottom of the conduction band in insulators and semiconductors. This is equivalent to the energy required to free an outer shell electron from its orbit about the nucleus to become a mobile charge carrier, able to move freely within the solid material. Intrinsic graphene is a semi-metal or zero-gap semiconductor. It has been discovered that a band gap can be produced in graphene by reducing the width of the material, particularly if the width can be reduced to a nanosized range. The lateral quantum confinement of the electron wave-functions and edge effects in the narrow GNRs, result in opening up of a finite energy gap. The semiconducting nature of sub-10 nm wide GNRs, both zigzag and armchair configurations, and the slightly wider armchair edged GNRs having widths of between 15-90 nm have shown potential for use in nanosized electronic devices. However, realization of this potential is dependent upon being able to produce GNRs with precise and reproducible dimensions at high throughput.
Several processes have been devised for GNR fabrication including lithography based methods, chemical and sonochemical methods, and the unzipping of carbon nanotubes (CNTs). However, achieving high throughput fabrication of GNRs with pre-determined widths using these processes is challenging. Lithographic methods of fabrication include on-substrate etching of a single large graphene sheet to obtain GNRs. Specifically, these lithographic methods include electron beam lithography, which has been used to create GNRs having a minimum width of 26 nm, and plasma etching with nano-rod masks and scanning-tunneling-microscopy. These lithographic methods have extremely low throughput.
GNRs can be produced through chemical and sonochemical methods such as through the ultrasonication of graphite in KMnO4 and H2SO4 solution and chemical vapor deposition from carbonaceous compound aerosols. The GNRs produced through ultrasonication of graphite are randomly sized and shaped, and the method provides no control on width or crystallographic orientation. Chemical vapor deposition provides limited control over GNR width (>20 nm) and no control on crystallographic orientation. Thus, both of these methods produce a broad distribution of GNR widths.
GNRs also can be produced through the unzipping of CNTs via chemical processes and through the etching of CNTs via an oxygen plasma process. The unzipping of CNTs via chemical processes produces GNRs at high-throughput. However, owing to the use of multi-walled carbon nanotubes (MWCNTs) in the unzipping strategy, the GNRs produced have a broad distribution of widths corresponding to the nanotubes' outer and inner circumferences. Further, because the unzipping process is oxidative in nature which leaves oxy-functional groups on the ribbons, production of pristine or non-functionalized GNRs cannot be realized through this method. The oxy-functional groups produce scattering sites on the graphene ribbons significantly reducing the carrier mobility, which is important for transistor applications of GNRs. Further, the oxy-groups change the sp2 hybridization of the carbons to spa, thus undesirably removing the pi-electrons density in GNRs. Etching of CNTs via an oxygen plasma process is a low throughput process.
Thus, there is a need for a method to enable production of controlled-width graphene nanoribbons at high-throughput.